[clang] [clang] Use FileError in FileManager::getFileRef, getDirectoryRef (PR #199126)

[Fangrui Song via cfe-commits]

MaskRay wrote:


There is a small compile time regression https://llvm-compile-time-tracker.com/compare.php?from=d2e9300b72637f92aa0ebf4aa5b82f84e4023640&to=b081351c7e20a26a6e67b3e82c065f02e4cce8d8&stat=instructions:u

```
	59234M (+0.04%)	74056M (+0.04%)	85564M (+0.04%)	17471M (+0.09%)	66487M (+0.03%)	20224M (+0.08%)	51324M (+0.03%)	15478M (+0.12%)	21519515M (+0.01%)
```

https://github.com/llvm/llvm-project/pull/199126